Embodiments of the inventive concept relate generally to semiconductor devices, and more particularly, to nonvolatile memory devices and methods of reading the same.
Semiconductor memory devices can be roughly divided into two categories, depending on whether they retain stored data when disconnected to power. These categories include volatile semiconductor memory devices, which lose stored data when disconnected to power, and nonvolatile semiconductor memory devices, which retain stored data even when disconnected from power.
Flash memory is a popular form of nonvolatile memory device. Flash memory is commonly used, for instance, in portable electronic devices, solid state drives, embedded systems and other devices where data storage is required even when power is lost or disconnected.
Two common types of flash memory include NOR-type and NAND-type flash memory devices. In NOR-type flash memories, at least two cell transistors are connected to one bitline in parallel. NOR-type flash memories store data using channel hot electron injection and erase data using Fowler-Nordheim (F-N) tunneling. NAND-type flash memories, on the other hand, include cell transistors connected to a bitline in series, and perform storage and erasing operations using F-N tunneling.
The memory cells in flash memory devices may each store one or more bits of data. To store one bit of data, a memory cell may use a variable threshold voltage that can be adjusted between two different levels corresponding to logic states ‘1’ and ‘0’. Similarly, to store two bits of data, the memory cell may use a variable threshold voltage that can be adjusted between four different levels corresponding to logic states ‘00’, ‘01’, ‘10’, and ‘00’. Likewise, memory cells having a variable threshold voltage can be used to store three, four, or more bits by changing the threshold voltage between different levels.